The present invention relates to a brushless motor which is energized by a poly-phase alternating current.
So-called radial gap type brushless motors with a radially running magnetic flux have been structured as in FIG. 5. The stator 23 has M.multidot.m teeth projecting toward a rotor 26 having m magnets 22, where M is the number of phases of the source current. In case of a three phase motor (M=3), for example, each exciting coil 21 is wound around two teeth of the stator 23 traversing two slots 25 (and one tooth 24) in between. Thus every coil 21 corresponding to the phase V of the source current traverses the other two coils corresponding to the other two phases U and W. The m sets of the three coils 21 are arranged around m permanent magnets 22 on the rotor 26 which include N and S poles placed alternately (therefore, the number m must be an even number).
The left column of FIG. 6 shows a change in the three phases U, V and W of the exciting current of the motor with respect to time. Each phase changes sinusoidally as time progresses (downward in FIG. 6) and is different by 120.degree. from the other two phases, which produces a rotating (leftward in FIG. 6) magnetic field, as shown in the middle of FIG. 6. The magnetic field produced by the coils 21 surrounds the rotor 26 and has m poles (N poles and S poles alternating) each corresponding to a pole of the magnets 22 on the rotor 26. Thus, the magnets 22 follow the rotating magnetic field, as the middle column sequentially shows from top to bottom, which produces the rotating movement of the rotor 26. The hatched curve on the fourth row of the middle column shows the magnetic field corresponding to the position of the rotor 26 as shown at the top of FIG. 6. The right column of FIG. 6 shows a change in the phase of voltage of the three coils induced by the movement of the magnets 22. The induced voltage is basically sinusoidal, but an irregularity is superposed thereon. The irregularity occurs when the boundaries between the N poles and S poles of the magnets 22 pass the slots 25 between teeth 24.
There are largely two drawbacks in the prior-art motors thus structured. First, the magnetic flux generated by a coil corresponding to one phase is partially canceled by opposite magnetic fluxes generated by the neighboring (and traversing) two coils corresponding to their respective phases at the traversing areas; this weakens the resultant magnetic field and decreases the efficiency of the motor. Second, the irregularity on the induced voltage causes a fluctuation in the output torque of the motor because the number of the magnets is the same as, or just n times, that of the exciting coils, so all the boundaries between the magnets 22 pass the slots 25 between teeth 24 simultaneously.
When the voltage induced in the coils becomes more closely sinusoidal, the rotation of the rotor becomes smoother. In the prior-art brushless motor, the ideal sinusoidal curve is difficult to obtain even when the shape of the magnets is properly modified. Such modification may instead decrease the output torque.